Archaeal uptake of enantiomeric amino acids in the meso- and bathypelagic waters of the North Atlantic

We determined the contribution of the three major prokaryotic groups (Bacteria, Crenarchaeota, and Euryarchaeota) on the uptake of Dand L-aspartic acid (Asp) in the major water masses of the North Atlantic (from 100to 4,000-m depth) with the use of microautoradiography combined with catalyzed reporter deposition fluorescence in situ hybridization (MICRO-CARD-FISH). The percentage of prokaryotic cells that assimilated Dand L-Asp ranged from ,5% to 25%. In the mesoand bathypelagic waters of the North Atlantic, Archaea are more abundant (42% 6 2% of 49,69-diamino-2-phenylindole [DAPI]–stained cells) than Bacteria (30% 6 1% of DAPI-stained cells), and more archaeal than bacterial cells are actively incorporating D-Asp (62% 6 2% vs. 38% 6 2% of total D-Asp active cells). In contrast, Bacteria and Archaea almost equally contribute to L-Asp use in the deep waters of the North Atlantic (47% 6 2% vs. 53% 6 2% of total L-Asp active cells). The increase in the D-Asp : L-Asp uptake ratio in the prokaryotic community with depth appears to be driven by the efficient uptake of D-Asp by, especially, the Crenarchaeota in the deep waters. Because Archaea, and particularly Crenarchaeota, commonly dominate the prokaryotic communities in the ocean’s interior, we suggest that they represent a previously unrecognized sink of D–amino acids in the deep ocean. The formation of the North Atlantic Deep Water (NADW) is the major driving force of the oceanic conveyor belt system that, in turn, influences the global climate (Broecker 1997). The turnover time of this oceanic conveyor belt system is about 2,000 yr, whereas that of the dissolved organic carbon (DOC) in the oceanic deep water is about 6,000– 8,000 yr (Williams 2000). Hansell and Carlson (1998) showed that the deep water DOC concentrations decline from the deep North Atlantic (;45 mmol L21) to the opposite end of the conveyor belt circulation, the deep Pacific (;37 mmol L21), indicating net removal of DOC. Despite recent advances in the phylogenetic characterization of deep-water prokaryotic communities, little is known about the metabolically active fraction of the prokaryotic community that drives the biogeochemical cycles in the 1 To whom correspondence should be addressed. Present address: Departamento de Ecologı́a y Biologı́a Animal, Universidad de Vigo, 36200, Vigo, Spain ([email protected]).